1. Field of the Invention
An aspect of the present invention relates to a thin film transistor and a method of fabricating the same, and more particularly, to a thin film transistor and a method of fabricating the same, capable of reducing stress of a substrate caused by a metal layer from source and drain electrodes, thus increasing yield of devices when manufacturing the thin film transistor of an organic light emitting diode display device.
2. Description of the Related Art
In general, an organic light emitting diode display device (OLED) includes an anode, an organic emissive layer disposed on the anode, and a cathode disposed on the organic emissive layer. In the OLED, when an electrical current is applied to the anode, a hole is injected into the organic emissive layer from the anode, and an electron is injected into the organic emissive layer from the cathode. The hole and electron injected into the organic emissive layer are recombined in the organic emissive layer to create an exciton, and when such an exciton transitions from an excited state to a ground state, light is emitted.
Generally, the organic light emitting diode display device is classified into a passive matrix type and an active matrix type depending on the manner of driving N×M number pixels disposed in the form of a matrix.
In the active matrix type, a pixel electrode defining an emissive region and a unit pixel circuit for programming current data or voltage data to the pixel electrode are disposed in a unit pixel region. Since the unit pixel circuit includes at least one thin film transistor, the unit pixel circuit supplies a constant current regardless of the number of pixels of the organic light emitting diode display device so that it provides stable luminance and low power consumption characteristics, thereby easily realizing a high-resolution large-size display.
FIG. 1 is a cross-sectional view illustrating a related method of fabricating a thin film transistor of an organic light emitting diode display device.
Referring to FIG. 1, a buffer layer 11 is formed on a substrate 10, and then a semiconductor layer 12 is formed on the buffer layer 11.
A gate insulating layer 13 is formed on the semiconductor layer 12, and a gate electrode 15 is formed on the gate insulating layer 13 to correspond to a predetermined region of the semiconductor layer 12.
Subsequently, impurity ions are injected into the semiconductor layer 12 using the gate electrode 15 as a mask, thereby forming source and drain regions 12a and 12b, and defining a channel region 12c interposed between the source and drain regions 12a and 12b at the same time.
An interlayer insulating layer 16 is formed on the entire surface of the substrate including the gate electrode 15 and then etched to form contact holes 17 exposing each of the source and drain regions 12a and 12b in the interlayer insulating layer 16.
Next, a metal layer for source and drain electrodes is deposited on the interlayer insulating layer 16 and then patterned to thereby form source and drain electrodes 18a and 18b contacting the source and drain regions 12a and 12b, thereby fabricating a thin film transistor.
FIGS. 2A and 2B are cross-sectional views illustrating a related metal layer for source and drain electrodes.
Referring to FIG. 2A, the source and drain electrodes are formed of a metal, such as molybdenum (Mo), tungsten (W), molybdenum-tungsten (MoW) or titanium (Ti), and these metals are formed by sputtering at a high temperature and crystallized to have a regular direction in the shape of a column.
Accordingly, after forming the source and drain metal layers using a metal such as molybdenum, as illustrated in FIG. 2A, the substrate is bent by stress caused by the change in thermal expansion coefficient due to orientation of the metal layer and temperature drop.
Such a phenomenon occurs frequently as the thickness of the substrate becomes thinner. Moreover, when a photoresist is coated on the metal layer to pattern the metal layer, the substrate may be broken.